Platinum-iridium clusters hydrogen

Data on the chemisorption of hydrogen at room temperature on platinum-iridium clusters dispersed on alumina and silica are shown in Figures 4.19 and 4.20 as a function of the amount of platinum and iridium in the catalyst (4). The data are for catalysts containing equal fractions by weight of platinum... 

For both total and strong chemisorption, H/M increases as the metal content of the catalyst decreases and is consistently higher for catalysts in which the platinum-iridium clusters are dispersed on alumina. As shown in Figure 4.19, the H/M values for total chemisorption frequently exceed unity. Values of H/M approaching 2 are observed at the lowest metal contents when the platinum-iridium clusters are dispersed on alumina. For strongly chemisorbed hydrogen, H/M appears to approach a limiting value near unity as the metal content is decreased to about 1 wt% or lower. 

If the alternative procedure of extrapolating the nearly pressure-independent region of the original adsorption isotherm back to zero pressure is employed, as discussed in Chapter 2, it is observed that the value of H/M for the alumina-supported platinum-iridium clusters in the catalyst containing 0.6 wt% metal is about 1.7. The strongly chemisorbed hydrogen determined by the method involving room temperature evacuation is approximately 75% of this value. 

When the platinum-iridium clusters are still more highly dispersed, and are supported on an alumina carrier instead of silica, the results shown in the right-hand sections of Figure 4.30 are obtained (48). The metal dispersion of the clusters as determined by hydrogen chemisorption is 0.93. If the clusters were spherical, the average diameter calculated from the chemisorption data would be about 12 A. Again the clusters are too small to give a satisfactory X-ray diffraction pattern. As with the previous two catalysts, the values... 

The selective production of methanol and of ethanol by carbon monoxide hydrogenation involving pyrolysed rhodium carbonyl clusters supported on basic or amphoteric oxides, respectively, has been discussed. The nature of the support clearly plays the major role in influencing the ratio of oxygenated products to hydrocarbon products, whereas the nuclearity and charge of the starting rhodium cluster compound are of minor importance. Ichikawa has now extended this work to a study of (CO 4- Hj) reactions in the presence of alkenes and to reactions over catalysts derived from platinum and iridium clusters. Rhodium, bimetallic Rh-Co, and cobalt carbonyl clusters supported on zinc oxide and other basic oxides are active catalysts for the hydro-formylation of ethene and propene at one atm and 90-180°C. Various rhodium carbonyl cluster precursors have been used catalytic activities at about 160vary in the order Rh4(CO)i2 > Rh6(CO)ig > [Rh7(CO)i6] >... 

Alternatively, extremely small clusters of platinum have been formed in zeolite LTL without a calcination step. Vaarkamp et al. [25] prepared platinum clusters of about 5 or 6 atoms each, on average, from Pt(NH3)4(N03)2 in zeolite BaKLTL simply by reduction at 500°C. Triantafillou et al [26] similarly prepared iridium clusters from [Ir(NH3)5Cl]Cl2 in zeolite KLTL by reduction in hydrogen at 300 or 500°C, also without calcination, the average cluster contained about 5-6 iridium atoms. 

Iridium clusters in zeolite KLTL, like the platinum clusters, consisting of 4 to 6 atoms on average, have also been prepared by hydrogen reduction of [Ir(NH3)5Cl]Cl2 in the pores at temperatures >300°C [26]. Even though the iridium clusters were as small as the selective platinum clusters in the same basic zeolite support, they were found to be unselective catalysts, being similar to other iridium catalysts for conversion of n-hexane and hydrogen principally into hydrogenolysis products. It is inferred that the combination of cluster size, electronic... 

Electron microscopy data on such catalysts show the average diameters of the metal clusters to be of the order of 10 A or lower. Clusters of this size necessarily consist almost exclusively of surface atoms. The stoichiometry of the strongly chemisorbed fraction thus appears to be close to one hydrogen atom per surface metal atom for platinum-iridium catalysts containing equal amounts of the two metals. If this stoichiometry were precisely correct, a value of H/M from Figure 4.20 would be a direct measure of the metal dispersion, that is, the ratio of surface atoms to total atoms in the metal clusters. 

A in size). On subsequent treatment in hydrogen at 500°C, the Ir02 crystallites are reduced to metallic iridium crystallites. The material then consists of a mixture of highly dispersed platinum or platinum-rich clusters and large crystallites of iridium. 

Figure 4.24 X-ray diffraction pattern showing the effect of calcining (heating) a silica-supported platinum-iridium sample in air at 500°C prior to reduction in hydrogen at 500°C, illustrating the importance of the preparative conditions in the formation of highly dispersed platinum-iridium bimetallic clusters (4). (Reprinted with permission from Academic Press, Inc.)...

Bimetallic clusters of platinum and iridium can be prepared by coimpregnating a carrier such as silica or alumina with an aqueous solution of chloroplatinic and chloroiridic acids (22,34). After the Impregnated carrier is dried and possibly calcined at mild conditions (250°-270 C), subsequent treatment in flowing hydrogen at elevated temperatures (300 -500°C) leads to formation of the bimetallic clusters. 

Although the Ira and Ir clusters catalyze the same reactions as metallic iridium particles, their catalytic character is different, even for structure-insensitive hydrogenation reactions. It is inferred [15] that the clusters are metal-like but not metallic consistent with the structural inferences stated above, we refer to them as quasi molecular. Thus these data show the limit of the concept of structure insensitivity it pertains to catalysis by surfaces of structures that might be described as metallic, i.e., present in three-dimensional particles about 1 nm in diameter or larger. This conclusion suggests that supported metal clusters may be found to have catalytic properties superior to those of conventional supported metals for some reactions. The suggestion finds some support in the results observed for platinum clusters in zeolite LTL, as summarized below. 

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